This invention relates to a transparent simulation disk used in place of a magnetic disk for measuring flying height, i.e., distance between a magnetic head and an information storage magnetic disk such as a hard disk or a removable disk for a computer.
In the semiconductor industry, there has recently been a rapid increase in recording density of a magnetic disk used as a hard disk. The increase in the recording density necessitates reduction in flying height of a magnetic head, i.e., distance between the magnetic head and the disk and nowadays the flying height is approaching a range of about 25 nm to 50 nm. Manufacturers of magnetic disks and manufacturers of hard disk drives conduct measurement of flying height by employing a method measuring interference of incident beam or by other method, using a transparent simulation disk instead of a magnetic disk. This transparent simulation disk consists merely of a substrate made of glass or quartz glass and, therefore, it has a high electric resistance and tends to be charged with electricity. As the accumulated charge increases, the Coulomb's force acting between the magentic head and the simulation disk (in this case the attraction force) increases as shown by the equation F=k.times.q.sub.1 .times.q.sub.2 /r.sup.2 (where F represents the Coulomb's force, k proportional constant, q.sub.1, q.sub.2 amount of electric charge, and r distance). By the Coulomb's force, an error tends to occur in the measured flying height and further increase in the electric charge causes even crash of the magnetic head against the surface of the simulation disk. If the magnetic head is brought into contact with the transparent simulation disk during a flying height test, the electric charge on the simulation disk flows as a current into the magnetic head and Joule heat generated by this current causes damage to the magnetic head which prevents continuation of the flying height test.
As described above, the recent tendency toward a high recording density of a magnetic medium demands a lower flying height. Since the Coulomb's force acting between the magnetic head and the simulation disk is in inverse prorpotion to the square of distance between the magnetic head and the simulation disk, there is an increasing tendency to occurrence of an error in measurement of flying height and crash of the magnetic head caused by the Coulomb's force produced by the electric charge accumulated on the simulation disk. Reduction of the electric charge therefore is a serious problem to be solved in the simulation disk.
In a prior art device disclosed by Japanese Patent Application Laid-open No. Hei 3-3177, for example, flying height of a magnetic head is measured by utilizing ultraviolet ray or visible ray. Since electric resistivity of a transparent simulation disk of this device is about 10.sup.15 -10.sup.16 .OMEGA.cm or over, electric charge is accumulated on the transparent simulation disk and damage of the magnetic head used in the flying height test is caused by charging of the accumulated charge. Since, moreover, voltage at the charging tends to decrease as the flying height decreases, damage rate of the magnetic head tends to increase. This problem is considered to be caused by the fact that the prior art transparent simulation disk has a very high value of electric resistance.
For reducing electric resistance of the surface of a simulation disk, it is proposed to form a conductive thin film on the simulation disk by employing a thin film forming technique including CVD, PVD, sputtering and vacuum deposition. This method is effective in reducing accumulation of electric charge by grounding this conductive thin film and thereby reducing damage to the flying height testing magnetic head occurring due to current flowing when the magnetic head comes into contact with the disk. This method is effective also for preventing accumulation of dust on the disk. This method, however, is disadvantageous in that the thin film reduces light transmittance and also the border between the thin film and the glass substrate exercises an adverse optical effect and, accordingly, an error in measurement tends to occur. Moreover, the thin firm is likely to come off when the magnetic head comes into contact with the disk.
It is, therefore, an object of the invention to provide a transparent similation disk for measuring flying height of a magnetic disk having sufficient conductivity in the substrate of the simulation disk and thereby being capable of preventing accumulation of dust and damage to the magnetic head caused by flow of current due to accumulated charge when the magnetic head comes into contact with the disk, having also transmittance and homogeneity equal to prior art transparent simulation disks (which have no conductive thin film) and a high resistance to abrasion.